The present invention relates generally to an apparatus and method for controlling electrical motors and generators, and more particularly, to an improved apparatus and method for controlling a synchronous electrical machine which selectively acts as both a motor and a generator by controlling phase angle.
A synchronous electrical motor can also act as a synchronous electrical generator, and therefore, devices which act as either a synchronous electrical motor or generator are referred to as synchronous electrical machines. Mechanical energy is changed into electrical energy by movement of a conductor through a magnetic field. The converse is also true wherein electrical energy is supplied to a conductor line normal to the magnetic field resulting in current flow in the conductor and mechanical force and thus mechanical energy being produced. Basically, the motor is the reverse of a generator; so simply stated, a synchronous motor is a synchronous machine that changes electrical energy into mechanical energy, and a synchronous generator is a synchronous machine that changes mechanical energy into electrical energy.
An alternator is a device that changes mechanical energy into alternating current to produce the electrical energy and is therefore an alternating current (AC) generator. On some applications the output of an alternator is rectified into direct current (DC) or in order to charge a battery or provide voltage and current (i.e., power) for a DC load.
A typical synchronous generator or motor, i.e., a synchronous machine, includes an armature core, an air gap, poles, and a yoke which form the magnetic circuit. The synchronous machine also includes an armature winding, a field winding, brushes and slip rings that form the electrical circuit. Further, the synchronous machine includes a frame, bells, bearings, brush supports and a shaft which together provide the mechanical support. The stator is generally the stationary component. The stator includes a group of individual electromagnets arranged in such a way that they form a hollow cylinder, with one pole of each magnet facing toward the center to the group. The rotor is the rotating electrical component, and the rotor includes a group of electromagnets arranged around a cylinder with the poles facing toward the stator poles. The rotor is located inside the stator and is mounted on the motor or generator shaft.
Another type of synchronous machine, a brushless version of the rotating magnetic field type, has armature coils for rotating the magnetic field in the stator and a field coil for the magnet in the rotor. Some synchronous machines utilize permanent magnets (PM) instead of electrically induced magnets to produce the magnetic moment of the rotor. PM type machines do not need electricity or slip rings, so such PM type machines are efficient in terms of energy consumption. However, PM type machines are generally expensive to produce and preclude the possibility of simple control of field strength needed when the synchronous machine under a varying RPM is to be both a motor and generator and under total control in either mode. Further, permanent-magnets are not permanent when inadvertently subjected to extreme high currents and extreme high temperature.
Other types of synchronous machines, which are really hybrid-type synchronous machines, include switched reluctance configurations with rotor position-triggered power electronics current control. The rotor position is either measured or estimated. The rotor position-triggered power electronics current control a variety of machines that are typically known as reluctance synchronous electric machines and drives with respect to rotor and stator configuration and stator current modes. Such reluctance synchronous machines have distributed anisotropy rotors formed of conventional or axial laminations with or without an equivalent cage winding in order to provide high saliency ratios from 6:1 to more than 20:1 which lead to high torque density, low loss, high power factor, and fast torque-speed transients. These types of hybrid-synchronous machines provide adequate improvements in control, however, they lack quick response and have difficulty in handling large KW needed for hybrid automobiles. They also lack a simple method of voltage control while in generating mode for braking.
Modern electric machines may generally have better efficiency and relatively reasonable costs in line-start applications and for power electronics and digital control in variable speed drives. As the cost of losses becomes higher and higher for line-start constant speed applications, high efficiency induction and synchronous motors are being produced at the cost of additional active materials (e.g., copper and iron).
Permanent magnets having high energy densities (˜35 J/cm3) are in existence and have been applied to synchronous, i.e., brushless, motors for high torque-density, low loss/torque, high KW/KVA ratio and fast torque and speed response drives with either sinusoidal or bipolar-rectangular current power electronics control. However, the relatively high costs of magnets, operating temperature limitations (100–150° C.) and the danger of PM demagnetization at high transient torques or at short circuit restrict the applications of such PM motors.
Vector control of induction motors provides almost equally fast speed dynamics with a lower cost rugged motor but with a slightly more complex and parameter-dependent controller and higher motor losses and static power converter ratings. Also, vector controlled induction motors are widely applied to spindle drives where a wide constant power (flux weakening) speed range is required. In applications where sustained large torque, low speed running is required, or in high precision machining (e.g., spindle) induction motor drives, the high rotor cage losses pose serious cooling and rotor thermal deformation problems, respectively.
Reluctance synchronous motors with distributed anisotropy rotors are made of conventional or axial laminations with or without an equivalent cage winding that provide high saliency ratios from about 6 to 1 to more than 20 to 1, where the higher values correspond to higher powers. Such reluctance synchronous motors have relatively high torque density, low loss/torque, high power factor, fast torque-speed transients and, for inverter-fed (cageless rotor) applications, to simplified control in the absence of rotor currents. The stator of these machines has uniform slots with concentrated (one tooth wide or pole-pitch wide coils (q=1)) with unipolar or bipolar two-level current control, or with distributed (q≧2) multiphase single or double windings and sinusoidal current (rotor-position-triggered) control. These types of pseudo-synchronous motors (essentially stepping motors) are lacking for all of the reasons set forth above including high heat loss in large KW sizes for hybrid automobiles, poor torque control at zero to very low RPM, difficulty controlling position especially while reversing and difficulty in controlling voltage while generating.
It is therefore desirable to provide a motor and generator (i.e., electrical machine) along with a control system that can control the phase angle of the motor with a linear voltage independent of frequency. It is also desirable to provide motor and generator control systems that can control the phase angle of the motor as well as torque from zero RPM and rotation in either direction and switch back and forth from motor to generator to motor within one half cycle of 3-phase power. It is also desirable to provide a synchronous machine configuration having high torque density, low loss/torque, high kW/KVA, fast torque and speed dynamics, wide speed range operation, efficient field weakening (i.e., constant power), motor ruggedness, high precision and robustness, and low motor costs. It is also desirable to provide a hybrid automobile having a synchronous machine as the primary drive control element. Further, it is desirable to provide a generator having a phase angle control device which can act as a cranking motor for automatic starting an internal combustion engine with automatic throttle control which provides engine RPM at maximum fuel efficiency for a corresponding KW load. Even further, it is desirable to provide an electrical machine transistor bridge wired in parallel to increase drive current maximums.